This invention relates to an hermetically sealed galvanic cell that has an improved terminal pin - collector plate assembly to facilitate venting of the cell.
Reliable, long service life cells or batteries have been developed for portable electrically powered devices such as tape recorders, playback machines, radio transmitters and receivers. Electrochemical cell systems for such devices provide a long service life by utilizing highly reactive anode materials such as lithium, sodium and the like, often in conjunction with high energy density nonaqueous liquid cathode materials and suitable salts, which are also referred to as cathode-electrolytes.
Galvanic cells typically are sealed to prevent loss of electrolyte by leakage. This is especially important in the case of nonaqueous liquid cathode cells, which typically employ highly reactive oxyhalide or halide cathode-electrolytes. Any escape of such liquids, or their reaction products, could cause damage to the device employing the cell, or to the surface of a compartment or shelf where the cell is stored.
On the other hand, certain operating conditions can cause the internal pressure of such liquid cathode cells to markedly increase. This pressure can be caused by external sources, such as fire, or internal sources, such as heat generated during charging. In certain situations, the anode can melt and react directly with the liquid cathode. In the case of other galvanic cells, such as alkaline-zinc cells, carbon-zinc cells, etc., such cells may enerate large quantities of gas under certain conditions of use. Thus, if any of the foregoing cells were permanently sealed, the build up of internal pressure within the cell could cause the cell container to leak, bulge or even rupture, which can cause property and/or bodily damage.
It is therefore necessary to provide a vent for galvanic cells that is designed to remain sealed during normal operating conditions which the cell may encounter, but which will open when the pressure within the cell substantially increases. In the case of liquid cathode cells employing, for example, a lithium anode, the vent must open before the lithium melts and reacts directly with the liquid cathode. Upon venting, most of the liquid cathode material is removed and is thus unavailable for reaction with the anode.
To meet these objectives, cells have been made with an electrically insulative frangible material, typically a glass or ceramic, disposed within and secured to a vent orifice that is typically located in the cell cover, so as to hermetically seal the vent orifice.
In one type of cell, referred to as a flat cell, a short, cylindrical container holds a wafer-like anode comprising an active anode material, such as lithium, disposed over and separated from a wafer-like cathode comprising an active cathode material, such as manganese dioxide.
A container cover disposed over and separated from the anode is hermetically sealed to the cell container. The cathode, which rests on the bottom of the cell container, is in physical (and thus electrical) contact with the container, so as to make the container the positive electrode of the cell. In contrast, the anode is electrically isolated from the container.
So that electrical contact can be established with the anode, a disk-shaped current collector plate is disposed over and placed in physical (and thus electrical) contact with the anode, and a collector insulator is placed between the current collector plate and the cover to maintain the electrical isolation of the anode from the container. A cylindrical pin is placed in electrical contact with the current collector plate and disposed to protrude through an orifice in the cell cover to form a negative electrode terminal. It is the typical practice to weld the cylindrical pin to the collector plate, so that the electrical resistance of the junction between the current collector and the cylindrical pin is low. An annular seal made of an electrically insulative frangible material is disposed within the orifice between the pin and the cell cover to hermetically seal the cell. When pressure within the cell reaches an unacceptably high level, the frangible material breaks, which should allow the cell to vent the excess pressure contained within it without rupturing.
Unfortunately, the construction of the cell can potentially interfere with safe venting. Upon the fracture of the frangible seal, the gases between the container cover and the collector plate quickly escape, while those between the collector plate and the bottom of the container encounter more resistance in their flow path and thus escape more slowly. In consequence, a pressure differential arises between the front and back of the collector plate, which urges the collector plate against the collector insulator, thereby partially resealing the cell. Thus, despite the frangible seal having fractured, there is no clear escape path for the gases within the cell, and the potential for destructive disassembly of the cell remains.